Cell Interactions during Growth and Morphogenesis 87 



mutations occur that prevent the mutant and its offspring from synthe- 

 sizing a capsule. The mutants are called R-cells, and since the colonies 

 derived from them possess an uneven outline and wrinkled appearance, 

 they are called "rough." 



If we inoculate a flask of nutrient broth with S-cells, we find that 

 by the time growth has ceased, the population is no longer composed only 

 of S-cells but also contains a great majority of R-cells. Figure 42 sum- 

 marizes the results. The total population grows normally until the fourth 

 day of incubation, then it dips quickly, begins to grow again, and finally 

 reaches a stationary state. Until the fourth day, the population is made up 

 almost completely of S-type cells. After 4 days, these decline rapidly. 

 R-cells make their appearance coincidentally with the decline of S-cells, in- 

 crease rapidly, and by 20 days make up over 90 per cent of the population. 



As the bacteria grow, intracellular substances leak into the broth; 

 one of these substances is the amino acid, alanine (CHs-CH-NHo'COOH). 

 The appearance of alanine coincides with the decline of the S-cells. The 

 S-cells are very sensitive to alanine and die in its presence, while R-cells 

 are resistant to alanine and can grow normally. Thus by the fourth day, the 



Fig. 42. Selective inhibition during growth of Brucella abortus cei 



2 4 6 



